A standard apparatus for making a tube has two grooved pilger rolls rotatable about respective axes to compress a metal tube moving in a travel direction against a mandrel extending in the direction between a nip formed between the grooved rolls. The mandrel is supported upstream of the nip in a thrust block and has a frustoconically tapered downstream portion extending through the nip so that as the tubular workpiece is compressed against the mandrel its diameter and wall thickness decrease. The rolls themselves are each formed with a substantially semicircular groove to impart the desired cylindrical outer surface to the workpiece.
Cold pilger rolling produces seamless tubes from a normally tubular metallic starting workpiece or blank to a finished product having cylindrical inner and outer surfaces. The purpose of pilger rolling is to reduce the outer diameter and wall thickness of seamless end product. The input stock here, known as the tube blank, is typically passed through a roll pair that has a conical pass design, and effects the rotational and feed motion intermittently on the tube blank. The rolling mandrel engages inside the tube blank.
When this approach is used, tubes are typically generated while maintaining especially stringent size tolerances of up to 5/100 mm. To perform quality control, the known approach has been to sample by removing and measuring tube samples after the forming process. Whenever the wall thickness threatened to depart from the tolerance range, or had already departed from this range, the rolling mill was shut down and the position of the rolling mandrel was corrected. However, this resulted in a situation where rapidly occurring changes in the wall thickness remained undetected and the rolling mill had to be regularly stopped for dimensional adjustment. Confirming that a dimensional adjustment had been successfully performed was also possible only after at least one more tube had been formed.
It would therefore be possible to perform nondestructive measurement of the forming result using, for example, conventional ultrasonic testing equipment. This approach is thwarted, however, by the especially small workpiece geometry and especially narrow tolerance specifications, and additionally by the lubricating film adhering to the workpiece and necessarily and unavoidably reaching the surface of the workpiece through the forming process.
U.S. Pat. No. 6,666,094 has already disclosed a method and an apparatus for effecting the noncontact online hot-wall-thickness testing of tubes. Here the impact of a pulsed laser against the wall of a hot-formed workpiece vaporizes not only the lubricating film adhering to the surface but also a small amount of the workpiece surface itself. The absorption of the laser energy within the tube surface and a partially effected vaporization of an extremely thin layer of the surface causes an ultrasonic pulse to be generated in the tube that enters the tube wall perpendicular to the tube surface. The ultrasonic pulse thereby generated is reflected from the inner surface of the tube back to the outer surface, is then reflected again, etc., with the result that an ultrasonic echo sequence of decreasing amplitude is created in the test material. The reflected ultrasonic pulse generates on the outer surface of the tube vibrations in the subminiature range that can in turn be detected in a noncontact procedure by a second laser operated in a continuous illumination mode by utilizing the Doppler effect.
The application of nondestructive testing methods for the cold-pilger-rolling process is, however, not known in the prior art. Instead, the testing methods used heretofore have continued to pursue the principle whereby measurement is performed after forming and sampling have been completed, individual or multiple forming parameters are then modified based on the test values, and finally the result of this parameter modification is then checked once again after a further completed forming process.